When combusting sulfur-containing fuels, the sulfur in the fuel is converted to sulfur oxides, mainly, sulfur dioxide (SO2), which may be harmful if emitted to the environment in large quantities. A well-known advantage of a circulating fluidized bed (CFB) boiler is the possibility to efficiently capture SO2 already in the furnace of the boiler by feeding CaCO3 containing absorbent, usually, limestone, into the furnace. At the temperatures prevailing in the furnace of a CFB boiler, usually, from 750° C. to 950° C., the CaCO3 calcines to calcium oxide (CaO), which reacts with the sulfur oxides to produce calcium sulfate (CaSO4), which can be removed from the furnace along with the ashes produced in the combustion.
In a circulating fluidized bed boiler, the bed is fluidized by introducing fluidizing gas through the bottom of the furnace by a relatively high nominal velocity, typically, from 3 m/s to 10 m/s. Because the upward flow of fluidizing gas entrains small-size particles of the bed, a CFB boiler comprises a particle separator that separates a portion of the entrained particles, generally, particles larger than a cut-off size of the particle separator, from the flue gas emanating from the furnace, to be returned back to the furnace. In order to efficiently utilize the limestone in the furnace, the limestone particles should be operational in the bed for a sufficient residence time. This is generally achieved by introducing into the furnace limestone that is crushed to a particle size that is large enough to form CaO particles that are larger than the cut-off size of the particle separator. Thereby, the CaO particles entrained with the flue gas are multiple times separated from flue gas in the particle separator and returned back from the particle separator to the furnace.
When SO2 reacts with the CaO particles, on the surface of the particles, forms a layer of dense CaSO4, which prevents the core of the particles from reacting with the SO2. Therefore, it is generally considered that there is an optimum size for the limestone particles, typically, 100 μm to 300 μm, to be fed to the furnace of a CFB boiler. The term particle size in this description refers to the median (d50) diameter of particle distribution as determined by a commonly used method, typically, by a laser diffraction based method. Even when using an optimum particle size, generally, an excess of limestone has to be fed into the furnace in order to obtain a desired level of SO2 removal. For example, in order to achieve 98% sulfur reduction efficiency in the furnace, typically, a Ca/S molar ratio as high as three to four is required.
When using high Ca/S ratios, such as from three to four, the bottom ash and fly ash discharged from the furnace invariably contain a large amount of CaO, typically, from about 10% to more than 50%, which makes the use or disposal of the ashes difficult. Because calcination of CaCO3 to CaO is an endothermic reaction, the calcination of excessive amounts of CaCO3 also decreases the thermal efficiency of the boiler. An advantageous method for enhancing sulfur reduction in a CFB boiler without feeding very high amounts of CaCO3 into the furnace is based on performing only a portion of the required SO2 reduction in the furnace, and complementing SO2 reduction in a semi-dry sulfur dioxide reduction device, such as a dry CFB scrubber or a spray dryer, arranged in the flue gas channel downstream of the furnace.
U.S. Pat. No. 7,427,384 suggests feeding calcium carbonate to the furnace of a circulating fluidized bed boiler so as to provide a Ca/S molar ratio of at most about 1.0 and further reducing the sulfur content of the flue gas by adding, for example, calcium hydroxide in a sulfur reducing stage arranged downstream of the furnace. Correspondingly, U.S. Pat. No 7,862,789 suggests conveying flue gases from a circulating fluidized bed boiler to a so-called flash dry absorber and introducing lime into the flash dry absorber.
Dry CFB scrubbers are commonly used for desulfurizing flue gases from combustion processes by using fresh hydrated lime as the sorbent. German patent No. 41 04 180 C1 teaches a method of desulfurizing flue gas of a CFB boiler by crushing and/or slaking CaO-containing fly ash particles separated from the flue gas of the boiler, and using the so treated particles as a sorbent in a dry CFB scrubber. U.S. Pat. No. 4,309,393 discloses a sulfur-reduction method for a fluidized bed boiler, in which CaO-containing ashes are collected from the furnace and slaked to form lime slurry for utilization in an SO2 stripping unit, such as a spray dryer, disposed in the flue gas duct downstream of the furnace. These methods improve the integration of the two desulfurization stages by utilizing CaO-containing fly ash in the second sulfur reducing stage downstream the furnace. A disadvantage of the methods is that they require additional measures and special equipment for treating the collected fly to a suitable form to be used in the second sulfur reducing stage.
An object of the invention is to provide an improved method of reducing sulfur dioxide content in flue gas emanating from a circulating fluidized bed boiler plant so as to minimize at least a portion of drawbacks of the conventional methods described above.